2x EL84 Fixed-Bias beyond 400V, anyone tried this?

[Vincebus Riveruptum]

Here is one running at 700V

Re: Power ratings... - Roger A. Modjeski - Tubes Asylum

DUUUDE

 

[duncan2]

Yes EL34,s Vincebus not EL84,s.
But that is not the limit the EL34,s can be run at 800 volts plate --400 volts screen for a load of 11000 ohms .


I just have a very hard time accepting that high a voltage for a STANDARD EL84 , I think there is more going on I don't know .

 

[Printer2]

Scott 299B. Not sure how it would like guitar duty, 430V, 420V at the plates. It also used the upgraded 6BQ5 - 7189. The picture does not seem to go full size so I included the link.

http://hhscott.com/pdf/sd/299B_3_4s.JPG

 

[Kay Pirinha]

Note the 7189's instead of 6BQ5's that we're discussing here!
Best regards!

 

[Printer2]

Yes, but some say a Russian 6P14P can be used as a substitute. I would just go to a bigger bottle but this is not what the thread asked for. Come to think of it, a 9-pin 12AB5 is used in a Harmon Kardon at 440V. I did a guitar amp with a pair running around 400V from memory.

 

[Vincebus Riveruptum]

sorry guys but i can't use any other tube rather the el84, currently i'm protoboarding and testing it with a bass, it sounds really nice... however i'm using a 450V transformer with a huge resistance(it drops to 380V at max vol). I will attach a demo in few mins

 

[Vincebus Riveruptum]

i got 24mA and the other 38mA(!) biased at -18V...this last one is a bit weak and the waveform shows up a bit assymetrical, later i will buy a matched pair

 

[PCL200]

No PCL200 ,the standard current draw for a pair of EL84,s in Class AB1 including the screen is ( approx) nearly 120ma , I always allow a safety margin as heating up transformers is not advised.

"Class AB1" and "nearly 120 mA" is vague. A pair could never idle (so no input signal) at 120 mA with Va = 450 V. So a reasonable explanation would be that you mean that the current at maximum power is nearly 120 mA.

So you at "nearly 120 mA" and me at "114 mA".

So than why the "No PCL200..."?

It looked to me that you suggested that the current demand on the power transformer at this anode voltage (450 V) is higher than at the more usual 300 V. That is, like I think you now also state, not the case.

The rest of the current demand depends on what else is running in the amplifier. A safety margin of 30 mA on top of that, looks unnessecary to me. But to each his own.

 

[Vincebus Riveruptum]

demo(sorry for the mic clipping)

 

[Printer2]

sorry guys but i can't use any other tube rather the el84, currently i'm protoboarding and testing it with a bass, it sounds really nice... however i'm using a 450V transformer with a huge resistance(it drops to 380V at max vol). I will attach a demo in few mins

380V is in the reasonable range for EL84's. Sounds like a bass.

 

[JMFahey]

IF the mic is clipping, record it again, otherwise the "demo" is useless, won´t even download it.
Sorry.

PS: are you recording with a cellphone?

 

[Vincebus Riveruptum]

380V is in the reasonable range for EL84's. Sounds like a bass.

yup is a bass

Measured correctly, at max vol the B+ drops to 415V, with 1KHz 2V sine wave at the input got 14V RMS with a 50W 8 ohm dummy load on the output, 24.5W... got 35W full clipping

 

[Vincebus Riveruptum]

IF the mic is clipping, record it again, otherwise the "demo" is useless, won´t even download it.
Sorry.

PS: are you recording with a cellphone?

yep i'm sorry , i don't have a good mic to record it so i used my phone, btw i'm going to record it again

 

[Printer2]

yup is a bass

Measured correctly, at max vol the B+ drops to 415V, with 1KHz 2V sine wave at the input got 14V RMS with a 50W 8 ohm dummy load on the output, 24.5W... got 35W full clipping

Meant that it was as expected. The sound is not that bad, one portion that was overloaded more than desirable but I thought it was usable in a live setting. Mind you depending on the setting.

 

[wiseoldtech]

You will probably find difference in power gives minimal increase in sound power.
For twice the loudness you need 10 times the power as the ear is logarithmic.
I would suggest at least a 100 watt amp.

Most musicians I know tend to prefer solid state amps if used for bass guitar's, etc.
That way, they can supply the punch they sometimes want during a gig.

 

[PCL200]

In the schematic in post # 2 there's no decoupling capacitor at the point where the screengrids are being fed. Is that on purpose, or just an omission?

 

[Vincebus Riveruptum]

it was on purpose to avoid overloading the screens making the voltage drop from 400v to a reasonable level depending on the input voltage. A capacitor will try to make the voltage constant thus melting the screens

 

[Vincebus Riveruptum]

i forgot to mention, i replaced R1 from 2.7k to 1K to increase the screen voltage from 260 to 300V, this increased the power output a bit enough to make 24.5W with less voltage, 425V

 

[PCL200]

The voltage drop over R1 (now 1K) is V4 (now 425 V) minus the sreengrid voltage (300 V) = 25 V. So the current through R1 = 25 / 1000 = 25 mA.

The voltage drop over R5 (22K) is 300 V - 273 V = 27 V. So the current through R5 = 27 / 22000 = 1.23 mA (which seems too low for the three 12AX7's that follow).

This would mean that the two screen grids of the EL84's are taking 25 mA - 1.23 mA = 23.77 mA. So each screen grid is taking about 11.89 mA. So the screen grid dissipation per EL84 is 0.0119 x 300 V = 3.57 Watt (well over Pg2 max, and just under Pg2 max peak).

If you look at datasheets for the EL84, a screen grid current at 300 V of almost 12 mA per EL84 at idle is way too high. It should be something like not more than 2 mA.

 

[Vincebus Riveruptum]

poor guy needs to go, is red plating now... here is a quick demo at max recorded directly to the pc with a cabinet simulator